Conventionally, an ink jet recording system is a recording system in which ink liquid droplets are ejected from a print head or a spray nozzle toward a material to be recorded, at high speed according to electronic signals from a computer or the like to perform printing on an ink receiving layer of the material to be recorded. In the above-mentioned ink jet recording system, the noise is small, and recording is possible at high speed, so that the system has widely prevailed in copies, facsimiles, posters, displays and the like. Based on this, it is required that an ink used in this ink jet recording system has no variation with time, and can maintain sufficient density for a long period of time when an image has been formed, and that the image printed on the material to be recorded has excellent original reproducibility and sufficient density.
For this reason, it is required that the ink used does not clog in the nozzle of a head, and does not clog a filter through which it passes in the course from an ink cartridge to the head. For the ink, therefore, it is necessary to adjust ink characteristics such as density, viscosity, particle size and concentration. The above-mentioned ink is generally constituted by a coloring agent such as a dye or a pigment, a dispersant or solvent for dispersing these, and an additive as needed.
In particular, a white ink for ink jet is printed on a low-brightness black-based or transparent material to be recorded, thereby obtaining printed matter having good visibility. It is therefore effective for marking of various products. Further, when the above-mentioned material to be recorded is printed with a colored ink other than the white ink, clear printing cannot be reproduced under the influence of a base color. For this reason, it is required that base printing is performed with a white ink having sufficient hiding properties to a ground to hide the blank ground, followed by printing on the white ground with the other colored ink.
However, in the above-mentioned white ink, a titanium oxide pigment is used as the coloring agent, and in the white ink in which a conventional titanium oxide pigment is used, redispersibility of the titanium oxide pigment becomes deteriorated due to the specific gravity of pigment particles, a solvent used and the like during storage, particularly, when an alcoholic solvent is used, by sedimentation and coagulation of the titanium oxide pigment with time. In order to solve such a problem, a white pigment composition (patent document 1) is proposed. However, in the white pigment composition disclosed in patent document 1, the redispersibility of a white pigment precipitated is improved by dispersing a titanium oxide pigment only treated with aluminum, in an alcoholic solvent using a carboxylic group-containing acrylic resin as a dispersant. In an actual operation, however, it has been desired that the composition can be used as such without performing redispersion of the pigment sedimented in the ink. Further, when the titanium oxide pigment is used as the white pigment, coagulation and sedimentation of the pigment in the ink are liable to proceed in storage of the ink. In particular, when it is used in an ink jet ink, the long period during which a printer is not used conceivably causes the case of unstable ejection or the case of clogging a nozzle of the printer, resulting in the necessity of replacing a head of the printer in the worst case.
Patent Document 1: JP-A-6-107964